1. Field of the Invention
The present invention relates to the art of casting aluminum killed molten steel and related ferrous alloys. The invention is directed toward tubes such as casting shrouds, nozzles (including submerged entry nozzles and submerged entry shrouds) and the like through which the molten metal passes during a continuous casting process. Typically these tubes are used in a continuous casting process for pouring the molten metal from a ladle into a tundish or from a tundish into a casting mold. The tubes of the present invention are made from a composition which is effective in preventing the deposition of non-metallic inclusions, especially alumina (Al.sub.2 O.sub.3), on the interior surface of the tube as the metal passes therethrough. In addition, the tubes made from this material also have a surprising thermal shock resistance. The invention is more particularly directed toward submerged entry nozzles and submerged entry shrouds which resist clogging caused by the deposition of aluminum oxide therein and which also have a surprising thermal shock resistance.
2. Background Information
It is well known that aluminum metal or alloys thereof may be added to molten steel in order to remove dissolved oxygen. The aluminum removes the oxygen from the steel by reacting with the oxygen to produce solid Al.sub.2 O.sub.3, most of which floats to the top of the molten steel where it can be easily removed. However, a small amount of Al.sub.2 O.sub.3 remains in the steel. The Al.sub.2 O.sub.3 which remains in the steel is known to accumulate and form a deposit on the inner surface of casting shrouds and nozzles as the molten metal passes therethrough. Although the reasons for this phenomenon are not completely understood, it is believed that the deposition occurs due to the presence of alumina in the refractory material of the nozzle which comes in contact with the molten steel containing residual alumina from the aluminum killing process.
The deposition of alumina is particularly troublesome in the nozzles and shrouds associated with a tundish which is used in a continuous casting process. In this type of process, the molten steel is teemed from a ladle through a nozzle or shroud into a tundish. The tundish includes a plurality of holes in the bottom which are connected to nozzles for the flow of molten steel therethrough into the casting machine. In order to accomplish this objective, it is important that the nozzles be able to provide a regular flow of molten metal to the casting machine. Typically, such casting machines operate at a specific casting rate. Obviously, it is important that the supply of molten metal which flows through the nozzles to the casting machine must remain as constant as possible during the casting procedure. Thus, nozzles which become partially or wholly occluded due to the deposition of alumina within the bore of the nozzle will cause serious problems in the casting procedure.
Various techniques are known in the prior art for avoiding the above-noted clogging problems. However, none of these have been totally satisfactory for a variety of reasons. For example, it is known in the art to provide a nozzle with a plurality of openings in the internal surface for the passage of an inert gas into the bore while the metal is flowing therethrough. In operation, gas is injected through these openings into the bore and this gas minimizes contact between the molten metal and the nozzle surface, thus preventing interaction between the metal and the nozzle which, in turn, prevents clogging from taking place. Typically, the openings constitute a highly porous surface which may be in the form of a porous sleeve within the bore of the nozzle. A nozzle of this type must include a complex and costly internal structure in order for the inert gas to reach the openings or pores within the internal portion of the nozzle. Thus, the manufacturing steps and costs associated with such a nozzle make this type of nozzle undesirable. In addition, the use of such nozzles is known to produce defects such as pinholes in the steel product due to the large amount of inert gas which is required to avoid the clogging problem.
Another approach to solve the clogging problem involves the fabrication of the nozzle from a material which inherently does not interact with the molten metal to form deposits of alumina. However, there are only a limited number of materials which are capable of functioning in this manner and which have the refractory properties which are needed in the environment of the molten metal casting apparatus. In particular, it is difficult to find a material which has the required thermal shock resistance needed for nozzles and the like through which molten metal flows.
U.S. Pat. Nos. 5,244,130; 5,046,647; 5,060,831 and 5,083,687 disclose various types of materials which are used to make nozzles and the like for casting molten metal. The specifications of each of the above-noted patents are incorporated herein by reference.
U.S. Pat. No. 5,244,130 (Ozeki et al.) provides an improved nozzle which is said to overcome the problems associated with other prior art nozzles. Ozeki et al. mention two types of prior art nozzles over which their invention is said to be an improvement. The first prior art nozzle is made from graphite and calcium zirconate (zirconia clinker) containing 23%-36% CaO. Ozeki et al. mention that the calcium oxide contained in the calcium zirconate does not sufficiently move toward the surface of the nozzle bore through which the steel flows and consequently the calcium oxide does not come into sufficient contact with the non-metallic inclusions such as .alpha.-alumina, and for this reason, this prior art nozzle is not effective in preventing the accumulation and deposition of alumina within the nozzle.
The second type of prior art nozzle discussed in U.S. Pat. No. 5,244,130 is similar to the first, but additionally includes calcium metasilicate (CaO.SiO.sub.2). It is said the that presence of the calcium metasilicate in the second type of prior art nozzle overcomes the problems noted with respect to the first type of prior art nozzle due to the combined effects of calcium zirconate and calcium metasilicate which allows the calcium oxide in each particle of zirconia clinker to move toward the surface. However, Ozeki et al. also note with respect to the second type of prior art nozzle that the calcium metasilicate has a low content of calcium oxide which is insufficient to adequately replenish the calcium oxide which reacts with the alumina in the molten steel; thus making it impossible to prevent clogging of the nozzle for a long period of time. In order to overcome this problem, Ozeki et al. use crystal stabilized calcium silicate (2CaO.SiO.sub.2 and 3CaO.SiO.sub.2).
The nozzles disclosed by Ozeki et al. include graphite in the amount of 10-35 wt. % which is added to improve oxide resistance, wetting resistance against molten steel and to increase thermal conductivity. Graphite in amounts which exceed 35% are avoided since such large amounts of graphite degrade corrosion resistance. There is no suggestion for adding flake graphite to improve the thermal shock resistance which is not surprising since the zirconia clinker used by Ozeki et al. is said to have a low thermal expansion coefficient.
U.S. Pat. No. 5,083,687 (Saito et al.) provides an improved nozzle for overcoming the above-noted clogging problem. Saito et al. mention that one type of prior art nozzle which was designed to avoid the clogging problem uses an inner lining made from a material containing 90-50 wt. % MgO and 10-50 wt. % C. However, it is noted in the specification that such materials containing graphite (C) and MgO suffer from cracking due to a large thermal expansion coefficient as compared to conventional nozzles made from alumina and graphite. Saito et al. also note that nozzles containing MgO and C exhibit inferior anti-spalling. In view of these undesirable features associated with refractories containing MgO and carbon, particularly the poor thermal shock resistance associated with the presence of MgO in the composition, Saito et al. concluded that nozzles which includes these ingredients would be unacceptable. Thus, Saito et al. avoid any material which contains MgO as a material for making the nozzle. Instead, they use a composition containing boron nitride, zirconium oxide and a sintering assistant containing SiC and B.sub.4 C.
U.S. Pat. No. 5,046,647 (Kawai et al.) discloses two types of improved nozzles for dealing with the clogging problem. One nozzle is made from ZrO.sub.2, C and SiO.sub.2. Kawai et al. emphasize that CaO and MgO should be avoided, or at best, can be tolerated in small amounts so that the sum of CaO and MgO is less than 1%. Kawai et al. also describe a second type of nozzle containing CaO and SiO.sub.2 in which the ratio of CaO to SiO.sub.2 is limited to 0.18 to 1.86. No MgO is disclosed for use in this second type of nozzle which is not surprising in view of the lack of thermal shock resistance noted in the prior art when MgO is included in the composition of the nozzle.
U.S. Pat. No. 5,060,831 (Fishler et al.) discloses a material for covering a casting shroud such as a tundish nozzle used for casting steel. The composition includes CaO and a zirconium oxide carrier. There is no suggestion for including MgO in the composition.